A method for concentrating sugars and decreasing fermentation inhibitor concentrations simultaneously before fermentation is disclosed. The method includes the steps of adjusting the pH value of a hydrozylate containing sugars and fermentation inhibitors; separating solid and liquid in the hydrozylate, for removing large particles in the solution; and applying a semi-permeable membrane, for concentrating the sugars and decreasing the total concentration of the fermentation inhibitors in the hydrozylate, so as to obtain a concentrated solution. By the aforesaid method, the sugar concentrations in the hyrozylate are increased, thereby increasing the concentration of ethanol in the subsequent fermentation. Therefore, the cost of purifying ethanol and the volume of the fermentation tank are both reduced. Moreover, the total concentration of the fermentation inhibitors, such as acetic acid, furfural and hydroxymethyl furfural (HMF), is decreased, favoring onset of fermentation.